hantek-6xxx: Fix coupling selection.

Due to an uninitialized variable, switching to/from AC/DC coupling
(on models that support this) was not working.

This fixes bug #836.

src/hardware/hantek-6xxx/api.c

@@ -120,6 +120,7 @@ static struct sr_dev_inst *hantek_6xxx_dev_new(const struct hantek_6xxx_profile
 	}
 	devc->coupling_vals = prof->coupling_vals;
 	devc->coupling_tab_size = prof->coupling_tab_size;
+	devc->has_coupling = prof->has_coupling;
 
 	devc->sample_buf = NULL;
 	devc->sample_buf_write = 0;